It depends on the sample size (diameter in mm?) I have seen tissue samples that are1-2 mm across, but also samples that fill the whole slide. It will also depend on the techniques you plan on using and whether you are using an automated stainer or not.

If you have time and your staining is not expensive . It would be good to optimize your conditions from 5-100microL, careful and dont get the slide completely dry. More the dye you use more the background